1. Field of the Invention
The present invention relates to a distortion correcting device for use at the neck of a stringed instrument such as a guitar, and more particularly, to a distortion correcting device in which the downward bow when the neck is distorted so as to be concave with respect to the strings and the upward bow when the neck is distorted so as to be convex with respect to the strings, is corrected.
2. Description of the Related Art
In a stringed instrument such as a guitar, strings are wound between a bridge mounted on the body, and a tuner mounted on the head which is at the front end of the neck. As a result, if the stringed instrument is stored with the strings being tensioned, the neck may distort into a concave configuration. When the neck bows downward, the length of the string is increased and playing of the instrument becomes difficult, and also when a string is depressed, the string tension is increased, thus causing the fret to be out of tune. In the past, various distortion correcting devices for correcting the downward bow of the neck have been proposed. For example, Japanese Utility Model Application Laid-Open No. 62-146195 discloses a distortion correcting device which includes an adjusting rod disposed inside a bar whose central portion has a U-shaped cross section, and the bar is nipped between one end portion of the adjusting rod which extends from the bar and an adjusting nut which screws together with the other end portion of the adjusting rod at the other end portion side of the bar. In this type of distortion correcting device, a closed space is accommodated between the groove and the finger plate formed in the neck. Also, when the neck bows downward, by the adjusting nut being tightened, the bar bends in the direction for upward bowing, and the downward bow in the neck is thereby corrected.
Also, there are guitars in which the neck is formed beforehand so as to have an upward bow and thus has a slightly convex portion, and when the strings are tensioned, the neck is distorted as a result of the string tension and the strings become parallel to the neck. As a result, the upward bow of the neck may become larger as a result of the quality of the seasoning of the material used for forming the neck, or as a result of changes in the environment subsequent to the manufacturing of the guitar. Thus, distortion of the convex configuration may still remain after the string has been tensioned. This distortion is an upward bow, and when this upward bow is formed, the string which is pressed during performance contacts the fret causing chattering and making performance impossible.
In the distortion correcting device disclosed by the applicant of the present invention in Japanese Patent No. 2,736,860, the downward bow and the upward bow of the neck can be corrected. In this distortion correcting device, one end portion of an adjusting rod is secured to a bottom portion of one end of a bar having a U-shaped cross-section, and an adjusting nut, which is screwed together with the other end portion of the adjusting rod, is rotatably supported by the other end portion of the bar. In this distortion correcting device, when the adjusting nut is tightened, the secured ends of the adjusting nut and the adjusting rod approach each other, and the bar bends back in the direction opposite to that of the downward bow of the neck. As a result, the downward bow of the neck is corrected. In addition, if the adjusting nut is turned in the direction for slackening, the distance between the secured ends of the adjusting nut and the adjusting rod is increased, and the bar bends back in the direction opposite to that of the upward bow of the neck, and as a result, the upward bow of the neck is corrected.
It should be noted that in Japanese Patent No. 2,736,860, a pressing element is provided at the adjusting rod so as to be positioned at the center of the distortion of the neck, and the distortion of the neck is effectively corrected by the bar being pressed by the pressing element.
However, in the above-described distortion correcting device, despite the fact that the pressing element is being used, when the bar is caused to bend back, it is difficult for the bar to achieve a suitable curvature, and it has become apparent that at the front end portion which secures the adjusting rod, the bar is not sufficiently bent back. In addition, there are some guitars in which the adjusting nut is accommodated at the head, and some in which the adjusting nut is accommodated at the base portion of the body side of the neck, and the type which is used is different depending on the guitar manufacturer or the type of guitar. In particular, in the guitar in which the adjusting nut is accommodated at the base portion of the neck, in order to ensure that the neck is sufficiently hard, the bar is sometimes not extended to the front end portion of the neck, and in this case, it is difficult for the correction of the distortion of the front end portion of the neck to be suitably carried out, and this problem needs to be solved.
An object of the present invention is to provide a distortion correcting device in which the curve of the bar is suitable after distortion is corrected, and the correction of the distortion can be accurately carried out.
A distortion correcting device for a neck of a stringed instrument, of the present invention comprises a bar accommodated at a concave portion of the neck, the bar having a bottom plate portion and a pair of opposing side plate portions erected from both sides of the bottom plate portion, an adjusting rod whose one end portion is secured to one end portion of the bar and whose other end portion engages with an adjusting nut which is rotatably supported at the other end portion of the bar, a first support member and a second support member which are secured to the side plate portions at both end portions of the bar, wherein the distance between the one end portion of the adjusting rod and the adjusting nut can be changed by turning the adjusting nut, and one end portion of the adjusting rod is supported such that movement in an axial direction is blocked by the first support member, and the adjusting nut is rotatably supported by the second support member.
In the distortion correcting device for the neck of a stringed instrument (simply referred to as “distortion correcting device” hereinafter), by turning the adjusting nuts, the distance between a first and second support member is changed, and the side plate portion which is secured by first and second support members extends or contracts in the longitudinal direction. On the other hand, because the length of the bottom plate portion does not change, a bend moment is generated at both end portions of the bottom plate portion by the length of the side plate portion being changed. The entire bars, including the bottom plate portion, is bent back by this bend moment. In this case, the bend moment which is generated at both end portions of the bottom plate portion acts on the support end portions of the adjusting rod and the adjusting nut which are at both the first and second support members. As a result, the size of bend moment of both is approximately the same. Accordingly, the bottom plate portion can be bent substantially uniformly and the distortion can be accurately corrected.
The side plate portions of the first and second support members are closely fitted into each other and may be fixed by being welded. However, if the first and second support members are fixed only by the welding the strength by which they are secured together and this is insufficient, it is thus preferable that the first and second support members be provided with convex portions which closely fitted into a concave portion formed in the side plate portions. Also, it is preferable that the concave portion and the convex portion be welded in a state in which they are fit into each other. In addition, the first support member is engaged in an axial direction with a peripheral edge groove formed in an outer periphery of one end portion of the adjusting rod, and the second support member is engaged in an axial direction with a peripheral edge groove formed in an outer periphery of the adjusting nut. Thus, the adjusting rod and the adjusting nut are firmly supported by the first support member and the second support member. Furthermore, the distances of the concave portions from the bottom plate portion are approximately equal, and thus the bend moment which acts on the respective end portions of the adjusting rod is approximately equal.
In the present invention, the adjusting nut is turned and caused to move relative to the adjusting rod, and thus it is necessary for the adjusting rod to be secured so that it does not rotate. In this case, the adjusting rod may be welded to the first supporting tool; however, by applying the structure described in the following, anti-rotation can be simplified and can be carried out at a low cost.
That is to say, at the front end surface of the adjusting rod, an anti-rotation concave portion which has a non-circular cross-section is provided, and the bottom plate portion is bent with respect to the front end edge of the bottom plate portion, thereby providing an anti-rotation plate portion which is equipped with a concave anti-rotation portion into which the convex anti-rotation portion fits. The cross section of the convex anti-rotation portion may have any suitably selected configuration, as long as it is not circular, and these include that of a circle having one or both sides cut out, or a polygonal configuration. In addition, because the anti-rotation plate portion is bent by sheet metal working, if the bottom plate portion and the side plate portion of the bar is formed integrally by a metal sheet being subjected to sheet metal working, the manufacturing cost is further reduced.
By carrying out anti-rotation in the manner described above without welding the adjusting rod to the first support member, the adjusting rod is movable in the direction which is perpendicular to the axial direction by the amount of the space between the convex anti-rotation portion and the convex anti-rotation portion. This is very important in order for the direction in which the bar is bent back to be secured and for the bending of the bar to be uniform. That is to say, when the adjusting rod is welded to the first support member, the center of the adjusting rod is sometimes mispositioned with respect to the center of the adjusting nut, due to error in manufacturing, or error in assembling or the like. If the adjusting nut is turned in this state, the bend moment acts on the adjusting rod, and this bend moment in turn acts on the bar.
Thus, it is not desirable for the front end portion of the adjusting rod to be firmly fixed by welding and the like, and it is desirable that a space be provided such that the adjusting rod is movable with respect to the bar in the direction which is perpendicular to the axial direction thereof. This space is suitably provided at both sides in the direction in which the adjusting rod moves toward or separates from the bottom plate portion (vertical direction) so as have sizes of 0.2 to 0.4 mm respectively, and to have sizes of 0.1 to 0.3 mm respectively in the direction which crosses that direction (lateral direction). The space which allows and also restricts movement of the adjusting rod in the above-mentioned direction is provided between the anti-rotation concave portion and the anti-rotation convex portion, and between the adjusting rod and the first support member and members, such as the bottom plate portion of the bar.